As one typical method of performing a performance evaluation on a data signal processing equipment or a transmission line, there has so far been proposed a method including the steps of inputting a data signal having a reference pattern as a test signal to an evaluation object, and measuring a bit error rate of a data signal outputted from the evaluation object.
The aforementioned method is realized by an error measurement apparatus comprising a waveform shaping circuit for removing an amplitude fluctuation from the data signal outputted from the evaluation object. The error measurement apparatus is designed to perform a bit state reading operation on the waveform-shaped data signal with a phase corrected clock to remove a phase fluctuation. In addition, the error measurement apparatus is designed to compare each bit state of the data signal without the amplitude and phase fluctuation with each bit state of the pattern of the test signal to be inputted to the evaluation object in order to obtain the error rate.
As previously mentioned, the conventional error measurement apparatus is designed to obtain the bit error rate after removing the amplitude and phase fluctuation from the data signal by the waveform shaping operation and the bit state reading operation with the clock signal. The conventional error measurement apparatus is, for example, disclosed in the following patent documents 1 and 2.    Patent Document 1: Japanese Patent Laid-Open Publication H5-7135    Patent Document 2: Japanese Patent Laid-Open Publication H8-88625